1. Field of Invention
This invention relates to apparatus and methods for the controlled polymerization of high pressure ethylene into polyethylene and to those types of processes in which temperature control is essential for assuring good quality as well as efficient and effective production.
2. Prior Art
While the invention relates generally to the controlling of processes in accordance with a peak or critical temperature which may shift back and forth in a reactor during a process, the invention will be best understood in terms of a preferred embodiment which is concerned with the polymerization of high pressure ethylene to produce polyethylene.
As set forth in U.S. Pat. No. 2,832,501 of Sept. 16, 1958 (W. R. Richard, Jr., et al.), polyethylene is an exceptionally important material of commerce suitable for use in molding and also having substantial use in film form. Polyethylene is most effectively produced by subjecting ethylene to the polymerizing action of elevated temperatures while the ethylene is confined within a tubular reactor or the like. The polymerization reaction is comparatively slow and it is known to employ an initiator solution which is introduced into the reactor to speed up the reaction. The solution includes catalysts such as, for example, benzoyl peroxide and various free-radical promoting catalysts. As will be seen, temperature peaks occur in reactors downstream of the position at which the initiator solution is introduced.
Some other patents relating basically to the production of polyethylene include U.S. Pat. No. 2,153,553 (Fawcett et al.) and U.S. Pat. No. 2,200,429 (Perrin et al.).
As further stated in U.S. Pat. No. 2,852,501, to obtain practical reaction rates and production yields, the ethylene catalyst mixture can be passed continually through a tubular reactor. Since the polymerization of ethylene is highly exothermic, conditions of temperature control are important and particularly peak temperatures which are reached within the reactor are of extreme importance. If too high a peak temperature occurs within the reactor, degradation of the product will result, this ranging in severity from discoloration of the polymer product to substantially complete carbonization of ethylene and polymer. For this reason it was sometimes believed preferable in the past to reduce various operational parameters to a lower value than would achieve an optimum yield in order to achieve a better quality of product. However, operating at milder reaction conditions does not necessarily assure trouble-free operation and sometimes degradation of the product can occur even at these reduced temperatures.
Pressure fluctuations occurring in reactors employed for the production of polyethylene result in temperature changes which also make the temperature difficult to monitor and control. Some of these pressure changes are incidental to reactions taking place during polymerization, but other pressure changes are purposefully employed to prevent the accumulation of polymer on the interior walls, these purposeful changes being known as "bump cycles" and being effected by the operation of "let-down" valves at the exit end of the reactor. This bump cycle may, for example, cause the reduction of pressure within a reactor from 40,000 psi to 35,000 psi, this being a drop of 5,000 psi which causes a shifting of the temperature profile throughout the reactor thus contributing to the difficulties experienced in monitoring peak or critical temperatures and controlling the associated process in accordance therewith.
Temperature profiles as mentioned hereinafter are discussed by way of example in U.S. Pat. No. 3,299,033 of Jan. 17, 1967 (R. M. Douglas). In this patent is discussed a method for continuously injecting a controlled volume of initiator solution through a line into a polymerization zone maintained at operating pressures in excess of 7,500 psi and in which zone the pressure is subjected to periodic pressure variations. The technique disclosed in this patent comprises applying and maintaining a pressure on the initiator solution in the line which is greater than the operating pressure existing in the reaction zone, continuously sensing the periodic pressure variations occurring in this zone, and continuously controlling the volume of initiator solution injected into this zone in response to the periodic pressure variations. This control is effective to increase the volume introduced during a pressure decrease in the reaction zone proportional to a resulting momentary increase in ethylene flow and is effective to decrease the volume introduced during a pressure increase in the reaction zone proportional to a momentary decrease in ethylene flow. This patent, moreover, discloses a control between an intensifying unit which supplies the initiator solution and the reactor into which the initiator solution is fed. It will appear from the description of a preferred embodiment of the present invention which follows hereinafter that an automatic control is provided for the operational speed of the intensifying unit by way of contrast to the control disclosed in U.S. Pat. No. 3,299,033 which, however, may be used in addition to the control of the invention.
Another intensifier is shown in U.S. Pat. No. 3,234,883 of Feb. 15, 1966 (R. M. Douglas et al.). This intensifier is a hydraulic intensifier system and provides for increasing the flow of initiator solution to a reactor by controlling the speed of an intensifier pump through the use of hydraulic pressure signals. The system shown in U.S. Pat. No. 3,234,883 may be employed in the apparatus of the instant invention, but the pressure signal which is employed to control the amount of initiator solution supplied to the reactor in accordance with the invention is generated in a unique and useful manner. Further discussion of intensifier systems and techniques is found in U.S. Pat. No. 3,234,882 of Feb. 15, 1966 (R. M. Douglas et al.).
In U.S. Pat. No. 3,079,372 of Feb. 26, 1963 (R. P. Fulknier et al.) is disclosed a system in which thermocouples are arranged at spaced intervals within a tubular reactor in temperature sensing contact with the contents of the reactor. Also provided is a product diversion valve which diverts product from a product collector. A valve actuator is provided which is responsive to the thermocouples to divert product when the temperature at any point in the reactor exceeds a predetermined level. A collector is disposed to receive the diverted product. As will be seen hereinafter, the invention detects a peak temperature in a reactor despite the positional shifting of the same and processes the signal to generate a basic signal to control the amount of initiator solution introduced into the reactor. This technique is of course wholly different from that disclosed in U.S. Pat. No. 3,079,372.
Other U.S. patents relating to the production of polyethylene include U.S. Pat. No. 2,870,130 of Jan. 20, 1959 (J. M. Davison et al.), U.S. Pat. No. 3,023,202 of Feb. 27, 1962 (M. M. Schappart), U.S. Pat. No. 3,053,640 of Sept. 11, 1962 (J. G. Kirkpatrick et al.), U.S. Pat. No. 3,236,828 of Feb. 22, 1966 (D. E. Carter et al.) and U.S. Pat. No. 3,293,233 of Dec. 20, 1966 (M. Erchak, Jr., et al.).